Millions of U.S. homes, 2,200 businesses such as Walgreens and 45 national landmarks including Seattle's Space Needle will go dark for an hour Saturday night to draw attention to global warming.

Despite debate over the science of climate change, a record number of U.S. cities, states, and businesses have pledged to participate in this year's Earth Hour, in which non-essential lights go off at 8:30 p.m. local time, according to organizers.

"It reflects the conviction of people around the world that climate change is real, and we need to do something about it," says Carter Roberts, chief executive officer of the World Wildlife Fund, which started Earth Hour in Sydney, Australia, in 2007.

The annual event has taken off since then. Last year, the group says, 4,000 cities turned off lights in official buildings in 87 countries, including 318 U.S. cities. It estimates 80 million Americans participated.

This year, 30 states (up from eight last year) plan to flip the light switch in governors' mansions and other public buildings. Among monuments and other landmarks taking part: the Empire State Building, the Golden Gate Bridge, the Las Vegas Strip, Niagara Falls and the Willis Tower, formerly the Sears Tower, in Chicago.

"As stewards of our national parks, we want to be visible leaders," says Navnit Singh, spokesman of South Dakota's Mount Rushmore National Memorial, participating for the first time.

Beginning Saturday, he says, the memorial will illuminate the mountain, in which the facial profiles of four presidents are carved, for only an hour a day in a bid to reduce energy use 60%. He says climate change has severely affected the Black Hills by increasing the number of mountain pine beetles, which have attacked 330,000 acres of ponderosa pine forest.

In Atlanta, where more than 650 public and private buildings will go dark, there will be a daytime festival to teach kids about Earth Hour and an evening dancing-in-the-dark party at the Savannah College of Art and Design, says Mandy Mahoney, the city's director of sustainability.

In the nation's capital, lights will go off in the Washington National Cathedral is also going dark. "As a community of faith, we recognize our planet as a sacred gift from God to enjoy, protect, and ultimately, pass on to the generations that follow," Samuel T. Lloyd III, the cathedral's dean, said in a statement.

Earth Hour has its critics

"Environmental challenges will not be solved by turning off our lights and symbolically hiding in the dark," says Michelle Minton of the Competitive Enterprise Institute, a Washington-based non-profit organization that promotes policies of free markets and limited government.

"We should be looking to technology and innovation to help solve environmental problems," she says. She'll attend her group's "Human Achievement Hour" cocktail party at 8:30 Saturday night to celebrate breakthrough products such as the compact fluorescent light bulb.

Roberts says he and his family will have a "candlelight" dinner and discuss how they can save energy. He's looking to insulate his house better and drive his car less. "For me," he says, "it means biking far more than I do now."